Exhaust gas emitted from an internal combustion (IC) engine, is a heterogeneous mixture that may contain gaseous emissions such as carbon monoxide (CO), unburned hydrocarbons (HC) and oxides of nitrogen (NOx) as well as condensed phase materials (liquids and solids) that constitute particulate matter. Catalyst compositions typically disposed on catalyst supports or substrates are provided in an engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
Exhaust gas treatment systems may include selective catalytic reductant (SCR) devices. The SCR device includes a substrate having a washcoat disposed thereon that operates to reduce the amount of NOx in the exhaust gas. The typical exhaust treatment system also includes a reductant delivery system that injects a reductant such as, for example, ammonia (NH3), urea (CO(NH2)2), etc.). The SCR devices make use of the reductant to reduce the NOx. For example, when the proper amount of reductant is supplied to the SCR device under the proper conditions, the reductant reacts with the NOx in the presence of the SCR washcoat to reduce the NOx emissions. However, if the reduction reaction rate is too slow, or if there is excess reductant in the exhaust, ammonia can slip through the SCR device and be emitted to the environment.